The invention is directed to an organic electroluminescent component, particularly an organic light-emitting diode.
The visualization of data is constantly increasing in significance due to the great increase in the amount of information. The technology of flat picture screens (xe2x80x9cflat panel displaysxe2x80x9d) was developed therefor for employment in mobile and portable electronic devices. The market of flat panel displays is currently largely dominated by the technology of liquid crystal displays (LC displays). In addition to cost-beneficial manufacture, low electrical power consumption, low weight and slight space requirement, however, the technology of LC displays also exhibits serious disadvantages.
LC displays are not self-emitting and can therefore only be easily read or recognized given especially beneficial ambient light conditions. This makes a back-illumination device necessary in most instances, but this multiplies the thickness of the flat panel display. Moreover, the majority part of the electrical power consumption of the display is then needed for the illumination, and a higher voltage is required for the operation of the lamps or fluorescent tubes, which higher voltage is usually generated from batteries or accumulators with the assistance of xe2x80x9cvoltage-up convertersxe2x80x9d. Other disadvantages are the highly limited observation angles of LC displays and the long switching times of individual pixels, which switching times typically lie at a few milliseconds and also are highly temperature-dependent. The delayed image build-up is considered extremely disturbing, for example, given utilization in means of conveyance or given video applications.
There are other flat panel display technologies in addition to LC displays, for example the technology of flat display panel cathode ray tubes, of vacuum-fluorescence displays and of inorganic thin-film electroluminescent displays. However, either these technologies have not yet achieved the required degree of technological maturity orxe2x80x94due to high operating voltages or, respectively, high manufacturing costsxe2x80x94they are only conditionally suited for utilization in portable electronic devices.
Displays on the basis of organic light-emitting diodes, which are called OLEDs, do not exhibit these disadvantages. The necessity of a back-illumination is eliminated due to the self-emissivity, as a result whereof the space requirement and the electrical power consumption are considerably reduced. The switching times lie at about one microsecond and are only slightly temperature-dependent, which enables employment for video applications. The reading angle amounts to nearly 180xc2x0, and polarization films that are required given LC displays are eliminated, so that a greater brightness of the display elements can be achieved. Further advantages are the employability of flexible and non-planar substrates as well as a simple and cost-beneficial manufacture.
The construction of organic light-emitting diodes typically ensues in the following way.
A transparent substrate, for example glass, is coated with a transparent electrode (bottom electrode, anode), composed, for example, of indium tin oxide (ITO). Dependent on the application, the transparent electrode is then structured with the assistance of a photolithographic process.
One or more organic layers composed of polymers, oligomers, low-molecular compounds or mixtures thereof are applied on the substrate with the structured electrode. Examples of polymers are polyaniline, poly(p-phenylene-vinylene) and poly(2-methoxy-5-(2xe2x80x2-ethyl)-hexyloxy-p-phenylene-vinylene). Examples of low-molecular compounds that preferably transport positive charge carriers are N,Nxe2x80x2-bis-(3-methylphenyl)-N,Nxe2x80x2-bis-(phenyl)-benzidine (m-TPD), 4,4xe2x80x2,4xe2x80x3-tris-N-3-methylphenyl-N-phenyl-amino)-triphenylamine (m-MTDATA) and 4,4xe2x80x2,4xe2x80x3-tris-(carbazole-9-yl)-triphenylamine (TCTA). Hydroxy-chinoline aluminum-III salt (Alp3) that can be doped with suitable chromophores (chinacridone derivatives, aromatic hydrocarbons, etc.), for example, is employed as an emitter. As warranted, additional substances, that influence the electro-optical and the long-term properties, such as copper phthalocyanine, can be present. The application of polymers usually ensues from the liquid phase with doctor blades or spin-coating; low-molecular and oligomeric compounds are usually deposited from the vapor phase by vapor deposition or xe2x80x9cphysical vapor depositionxe2x80x9d (PVD). The overall layer thickness can amount to between 10 nm and 10 xcexcm and it typically lies in the range between 50 and 200 nm.
A cooperating electrode (top electrode, cathode), which is usually composed of a metal, of a metal alloy or of a thin insulator layer and a thick metal layer, is applied onto the organic layer or layers. The manufacture of the cathode layer usually ensues with vapor phase deposition by means of thermal evaporation, electron beam evaporation or sputtering.
When metals are employed as cathode material, then these must have a low work function (typically  less than 3.7 eV) so that electrons can be efficiently injected into the organic semiconductor. Alkaline metals, alkaline earth metals or rare earth metals are usually employed for this purpose and the layer thickness lies between 0.2 nm and a few hundred nanometers but generally at a few 10 nanometers. Since, however, these non-precious metals tend toward corrosion under atmospheric conditions, it is necessary to additionally apply a layer of a more precious, inert metal such as aluminum (Al), copper (Cu), silver (Ag) or gold (Au) onto the cathode layer that protects the non-precious metal layer against moisture and atmospheric oxygen.
For increasing the stability of the cathodes against a corrosion-caused hole formation, an alloy composed of an efficiently electron-injecting but corrosion-susceptible non-precious metal (work function  less than 3.7 eV) and a corrosion-resistant or precious metal, such as Al, Cu, Ag and Au, is often employed instead of an unalloyed non-precious metal. The proportion of the non-precious metal in the alloy can amount to between a few tenths of a percent and approximately 90%. The alloys are usually generated by simultaneous deposition of the metals from the vapor phase, for example by co-vapor deposition, simultaneous sputtering with a plurality of sources and sputtering upon employment of alloy targets. However, a layer of a precious metal or corrosion-resistant metal, such as Al, Cu, Ag or Au, is usually also additionally applied onto such cathodes as protection against corrosion.
Cathodes composed of precious metals, i.e. metals having a work function  greater than 3.7 eV, are very inefficient electron injectors when they are utilized in direct contact with the organic semiconductor. When, however, a thin insulating intermediate layer (layer thickness generally between 0.2 and 5 nm) is arranged between the uppermost, electron-conducting organic layer and the metal electrode, then the efficiency of the light-emitting diodes rises substantially. Oxides such as aluminum oxide, alkaline and alkaline earth oxides and other oxides as well as alkaline and alkaline earth fluorides come into consideration as the insulating material for such an intermediate layer (in this respect, see Appl. Phys. Lett., Vol. 71 (1997), pages 2560 through 2562; U.S. Pat. No. 5,677,572; European Published Application 0 822 603). A metal electrode that is composed of a pure metal or of a metal alloy is then applied onto the thin, insulating intermediate layer. The insulating material can thereby also be applied together with the electrode material by means of co-vapor deposition (Appl. Phys. Lett., Vol. 73 (1998), pages 1185 through 1187).
An object of the invention is to fashion an organic electroluminescent component, particularly an organic light-emitting diode, such that, on the one hand, a hermetic seal of the top electrode can be foregone and, on the other hand, the selection of materials employable at the cathode side is greater.
This is inventively achieved by a component that is characterized by or comprises
a transparent bottom electrode situated on a substrate;
a top electrode composed of a metal that is inert to oxygen and moisture;
at least one organic function layer arranged between the bottom electrode and the top electrode; and
a charge carrier injection layer containing a complex metal salt of the composition (Me1)(Me2)Fm+n, whereby the following applies:
m and n are respectively a whole number corresponding to the valence of the metals Me1 and Me2 (the metal Me1 thereby has the valence m, the metal Me2 the valence n),
Me1 is selected from a group consisting of Li, Na, K, Mg and Ca.
Me2 is selected from a group consisting of Mg, Al, Ca, Zn, Ag, Sb, Ba, Sm and Yb,
with the prescription: Me1xe2x89xa0Me2.
The critical feature of the organic electroluminescent component of the invention is thus in a specific structure at the cathode side, namely in the combination of a top electrode that is indifferent with respect to environmental influences with a charge carrier injection layer composed of a specific complex metal salt having the composition (Me1)(Me2)Fm+n, i.e. a double fluoride. As a result of this structure, a hermetic seal or, respectively, sealing of the top electrode can be omitted. As a result of the specific material for the charge carrier injection layer, not only is the offering for the materials employable at the cathode side broadened, this material also achieves an improvement of the emission properties, which are expressed in clearly higher light yield, a reduced operating voltage and a longer service life during operation.
The charge carrier injection layer (composed of a specific complex metal salt) is preferably arranged as a thin insulating layer either between the top electrode and the organic function layer or between the uppermost function layer and the top electrode given the presence of a plurality of function layers. When an electron transport layer is also additionally located on the (uppermost) function layer given the component of the invention, then the charge carrier injection layer is arranged between this transport layer and the top electrode. In all of these instances, the thickness of the charge carrier injection layer preferably amounts to approximately 0.1 through 20 nm.
However, the charge carrier injection layer can also be quasi-integrated into the top electrode, into the (uppermost) organic function layer or into an electron transport layer that is potentially present, i.e. the complex metal salt is then a constituent part of one of these layers. The production of such layers can advantageously ensue by means of co-vapor deposition of the corresponding materials, for example by co-vapor deposition of the top electrode material and of the complex metal salt.
The complex metal salt exhibits the composition (Me1)(Me2)Fm+n, whereby m and n correspond to the valence of the respective metal. m=1 (Li, Na, K) or m=2 (Mg, Ca) is valid for Me1; n=1 (Ag) or n=2 (Mg, Ca, Zn, Ba) or n=3 (Al, Sb, Sm, Yb) is valid for Me2. The metal Me1 is preferably lithium (Li); the metal Me2 is preferably magnesium (Mg), aluminum (Al), calcium (Ca), silver (Ag) or Bariumn (Ba).
Advantageously, one of the double fluorides LiAgF2, LiBaF3 and LiAlF4 is employed as the complex metal salt. More such double fluorides are, for example, NaAgF2, KAgF2, LiMgF3, LiCaF3, CaAgF3 and MgBaF4. Complex salts of this type as well as methods for manufacturing them are known in and of themselves (in this respect, see the exemplary embodiments as well as, for example, xe2x80x9cGmelins Handbuch der Anorganischen Chemiexe2x80x9d, 8th Edition (1926), System Number 5 (fluorine), pages 58 through 72).
The top electrode, which generally comprises a thickness greater than 100 nm, is preferably composed of one of the following metals: aluminum (Al), silver (Ag), platinum (Pt) and gold (Au). The electrode material, however, can also be an alloy of two of these metals. Magnesium (Mg), calcium (Ca), zinc (Zn), antimony (Sb) and barium (Ba) come into consideration as other metals for the top electrode.
The bottom electrode is generally composed of indium tin oxide (ITO). Other possible materials for the bottom electrode are tin oxide and bismuth oxide. Glass generally serves as the substrate for the bottom electrode.
The component of the invention preferably comprises two organic function layers, namely an apertured conducting layer arranged at the bottom electrode that transports positive charge carriers and an emission layer situated thereon that is also referred to as the luminescence layer. Two or more apertured conducting layers can also be advantageously utilized instead of one apertured conducting layer.
The materials for these layers are known in and of themselves. In the present case, N,Nxe2x80x2-bis3-methylphenyl)-N,Nxe2x80x2-bis(phenyl)-benzidine (m-TPD), 4,4xe2x80x2,4xe2x80x3-tris-(N-1-naphthyl-N-phenylamino)-triphenylamine (naphdata) or N,Nxe2x80x2-bis-phenyl-N,Nxe2x80x2-bis-xcex1-naphthyl-benzidine (xcex1-NPD) is preferably employed for the apertured conducting layer or layers. The material for the emission layer is preferably hydroxychinoline aluminum-III salt (Alq3). Simultaneously, this material can also serve for the electron transport. For example, chinacridone can also be utilized for the emission layer, and one of the oxadiazole derivatives known for this purpose for a potentially present electron transport layer.
The invention offers the following, additional advantages, particularly in view of organic light-emitting diodes:
Facilitated Handling
Due to the stability of the material of the top electrode, work need not be carried out under an inert gas atmosphere in the manufacture and further-processing of OLEDs.
Performance
Compared to top electrodes of non-precious metals, the operating voltage is clearly lowered and the light yield and efficiency are considerably enhanced.
Improved Properties
Compared, for example, to LiF as the material for the intermediate layer, compounds such as LiAlF4 have the advantage that they are less hygroscopic, which facilitates the handling and storage. The double fluorides are also easier to evaporate and are less basic than LiF, as a result whereof the compatibility with the organic function layers is increased.
The invention shall be explained in still greater detail on the basis of exemplary embodiments and Figures.